Recent cold and potential damage, plus apple scab management.

By Terence Bradshaw

11 April 2016

If it weren’t for that warm spell, everything would be all right. Orchards around Vermont are largely at the ‘advanced silver tip’ bud stage and have been since the first of April, with tighter buds on Honeycrisp and in cooler upland orchards and more advanced buds at green tip on early cultivars and in warmer areas. Recent cold temperatures, especially on April 5, when we saw 10°F in South Burlington, are cause for concern, but don’t write off the crop yet. Critical temperatures for bud damage to apples at silver tip are 15°F for 10% kill and 2°F for 90% kill as a rule of thumb, although many factors can adjust those numbers. If buds were more advanced to the green tip stage, then the 10% and 90% kill temperatures are 18 and 10°F, respectively. I have only looked at a handful of buds at the UVM orchards, and saw a few with browning in the flower primordia. I prefer to take a wait and see approach because I just don’t trust evaluations yet, I don’t feel the damage was too extensive, and we still need to manage our trees for disease in the early spring regardless of whether there is a crop or not.

Apple scab management is right around the corner, and when the warm temperatures arrive by the weekend we can expect rapid bud stage development and plenty of green tissue exposed that will need protection. Many orchards has issues with late-season apple scab in 2015 and inoculum may be on the high side, so it would make sense to keep things covered during early scab infections this year. Luckily, the cool/cold weather has kept ascospore maturity low, with NEWA estimating 2% maturity in South Burlington and a predicted 3% of spores mature by the weekend. However, warmer sites like Putney are predicting maturity of 5% by this weekend, and if inoculum is high, 2-5% can cause substantial infection. Rains are expected for next Tuesday, and while I wouldn’t put too much faith in an eight-day forecast, I would plan on protecting trees as soon as possible after green tip emerges. This likely means spraying Friday and through the weekend. Remember to avoid using phytotoxic materials (coppers, captan, oil) before or after a freeze. Copper would be preferred for its efficacy against fire blight at this stage as it’s probably the best and maybe only window to apply it before 1/2” green bud stage comes and chances for phytoxicity and fruit russeting increase. This may also be a good window for applying oil so long as you wait at least 24 and preferably 48 hours after a freeze event to avoid damaging developing tissues. As always, materials and rates are best found in the 2015 New England Tree Fruit Management Guide, please let me know if you need a copy.

Weather looks great for planting trees, so get them in the ground this week before you get pulled away with spraying later.